First, cut the onion in half and slice it lengthways. Make one cut horizontally through the onion, and then turn the onion round, draw back the knife and slice downwards, being careful to keep your fingers behind the blade.
Chopping an onion through releases acidic juices, which can result in a bitter taste to your sauce. Haworth suggests wrapping the onion in a cloth and washing it under running hot and cold water several times, finishing with cold water. Then squeeze out the water and you will be left with fresh onions without a bitter taste.